A cognitive radio of recognizing an ambient wireless environment and optimizes communication parameters according to the wireless environment has been known. As an example of a cognitive radio, for example, there are cases in which a secondary system (interfering system) shares a frequency band allocated to a primary system (interfered system).
When the secondary system shares the frequency band with the primary system, the secondary system does not have to influence existing services provided by the primary system. To this end, a transmitting station (hereinafter referred to as a “secondary transmitting station”) of the secondary system performs communication with transmission power (hereinafter referred to as “permissible transmission power”) which is adjusted so that a certain reception quality can be maintained in a receiving station (hereinafter referred to as a “primary receiving station”) of the primary system. Here, as a reference used to maintain a certain reception quality, a technique of maintaining a carrier to interference ratio (CIR) or a carrier to interference plus noise ratio (CINR) of the primary receiving station to be equal to or more than a certain value or a technique of maintaining an interfered amount of the primary receiving station to be equal to or less than a certain value is considered.
In Non-Patent Document 1, permissible transmission power for maintaining the CIR to be equal to or more than a certain value in a receiving station of a television broadcast system serving as the primary system is discussed. In the technique disclosed in Non-Patent Document 1, the permissible transmission power is decided by estimating a path loss (propagation loss) when a signal (secondary signal) transmitted from a secondary transmitting station arrives at a primary receiving station and estimating the interfered amount of the primary receiving station caused by the secondary signal. However, since the path loss estimation includes an error caused by shadowing, an error caused by a difference between a propagation model (a path estimation formula such as the Okumura-Hata formula) and a real environment, or the like, a CIR estimation error occurs consequently. For this reason, in Non-Patent Document 1, a margin according to a CIR error value is added, and then the permissible transmission power is set. As a result, by restricting the permissible transmission power according to the CIR estimation error value, the CIR can be maintained at a certain probability.
Further, Patent Document 1 discloses a technique in which a base station device of a wireless communication system shares a common and/or adjacent frequency band with a second wireless communication system, obtains a separation frequency width between a frequency band to be used and a frequency band to be used by the second wireless communication system, obtains a separation distance between the base station device and a receiving device equipped in the second wireless communication system, and decides maximum transmission power that does not cause the communication quality of the second wireless communication system to deteriorate which is a priority system based on the separation frequency and the separation distance. In the technique disclosed in Patent Document 1, the separation distance and the estimation propagation loss amount are calculated using a free-space propagation loss formula. In the case of the technique disclosed in Patent Document 1, similarly to the technique disclosed in Non-Patent Document 1, since an error caused due to a difference with a real environment is included, a CIR estimation error occurs consequently.
Meanwhile, Non-Patent Document 2 discloses interference monitoring in which a monitoring station (in Non-Patent Document 2, described as a secondary receiving station of a secondary system) positioned around a primary receiving station measures a reception level of a secondary signal or a reception level of a primary signal transmitted from a primary transmitting station of a primary system, and uses the measured reception level for CIR estimation in the primary receiving station. In this technique, an estimation value of a path loss used in the technique disclosed in Non-Patent Document 1 is actually measured and corrected using the reception level of the secondary signal or the primary signal measured by the monitoring station. Through the correction of the estimation value of the path loss using the measurement result, the CIR estimation error can be reduced, a margin necessary for suppression of the permissible transmission power can be reduced, and thus the permissible transmission power can be increased. Further, since the monitoring station is used for measurement, there is an advantage of improving the CIR estimation accuracy in the primary receiving station without changing the receiving station of the primary system which is the priority system.